In drilling oil and gas wells and the like by the rotary drilling method, a string of drill pipe having a drill bit mounted on the lower end thereof is rotated to cause the cutting elements or "teeth" on the bit to drill the hole. Drilling fluid is circulated through the drill pipe emerging through openings in the drill bit returning to the surface in the annular space between the drill pipe and the walls of the borehole. Such circulation is substantially continuous while drilling, being interrupted by essential operations, such as adding an additional section of drill pipe at the top of the drill string or when the entire string of drill pipe is pulled from the wellbore to replace a worn-out drill bit.
In addition to removing drill cuttings from the hole the drilling fluid performs many functions vital to a successful drilling operation. These functions have been discussed by Rodgers ("Composition and Properties of Oil Well Drilling Fluids," Water F. Rodgers, pps. 10-18, Gulf Publishing Company, Houston, Texas, 1963).
In the drilling of wells sand contamination of drill fluid results from the drilling of sandy shales and sandstones. Sand is a highly undesirable mechanical contaminant because sand is considerably harder than most steels and its abrasive qualities cause rapid and excessive wear of pipe elbows and reciprocating and centrifugal pumps. Higher sand contents in drilling fluids increase drill pipe friction resulting in increasing rotating torque and drag. The rapid settling of sand may even stick drill pipe in the hole or cause core barrels to fail to operate. Sand may also bridge outside the casing and prevent a satisfactory cementing operation. For these reasons, drilling operators make every reasonable effort to keep sand out of drilling fluids; sand contents of 2 or 3 percent can be tolerated but if the percentage rises, steps must be taken to reduce the sand content.
Crooked holes, including doglegs, corkscrews, and boreholes deviating from the vertical are a primary cause of excessive torque and drag.
It is important to recognize that ordinary crooked holes can be significantly minimized by good drilling engineering practice. However, current economic and environmental considerations dictate that an increasing number of wells, both on land and offshore, be deliberately deviated. In such deviated holes where the borehole is at an angle from the vertical, the drill string must rest against the side of the wellbore. This lateral force significantly increases normal rotating torque and drag over that of vertical drilling, notwithstanding whether the drilling fluid is oil base or water base. This frictional effect becomes very important in current offshore drilling where 20 to 22 wells are drilled from a single platform and the deviation of the wellbore from vertical is sixty degrees or more.
The rate at which the hole can be made depends in part upon the rate of rotation of the drill pipe and upon the "weight on bottom" or force with which the drill bit acts on the bottom of the hole. This force is controlled by addition to drill collars which are pipes of larger diameter and greater mass than drill pipe. It is, therefore, very desirable to minimize friction upon the drill string and maximize horsepower at the bit.
Clearly, high rotational friction or high drag friction in removing a string of drill pipe miles in length for the purpose of periodically changing bits can severely limit the ability and efficiency of a given derrick to drill deep wells and also increases the cost of drilling.
In present day offshore drilling there are areas where basic rig overhead costs are $50,000 or more per day. It is, therefore, economically desirable to drill as rapidly as possible with minimum equipment and power.
Current drilling fluid technology predicated upon maintaining low clay solids in aqueous drilling muds also contributes to increasing rotating torque and pipe drag. Traditional drilling muds contained an appreciable component of hydrated bentonite which acted as a borehole lubricant. With the advent of solids control equipment and the deliberate reduction in bentonite content for the purpose of increasing penetration rate and minimizing formation damage, this lubricating effect of bentonite has been greatly reduced.
Even with traditional bentonitic drilling fluids and techniques, the friction of running pipe into and out of the hole, the increases in torque and power to rotate the drill pipe, the wear and stress on drill pipe and danger of twist offs of the drill pipe has caused numerous drilling fluid lubricants to be investigated.
The prior art shows such lubricant drilling additives to be composed of saturated or unsaturated fatty acids, mixtures of fatty acids and resin acids, naturally occurring triglycerides, stearates of aluminum and other metals, fatty amides, sulfurized vegetable oils, sulfated fatty acids and fatty alcohols and mixtures thereof and their solutions or emulsions in water or primary alcohols of 12 to 15 carbon atoms.
In general all sorts of soft solids including graphite, blown asphalts, gilsonite, soaps, plastics (such as polyethylene or Teflon particle dispersions), have been proposed as drilling fluid lubricants. A wide variety of such substances that have a known performance history as boundary or hydrodynamic lubricants in industry have been introduced as drilling fluid lubricant additives, as for example in U.S. Pat. Nos. 2,773,030; 2,773,031; 3,014,862; 3,027,324; 3,047,493; 3,047,494; 3,048,538; 3,214,374; 3,242,160; 3,275,551; 3,340,188; 3,372,112; 3,377,276; and 3,761,410.
Ground walnut hulls are commonly used in drilling fluids for lost circulation control. It is, however, a matter of common knowledge that when angular walnut hull fragments are introduced into a circulating drilling fluid that some reduction in rotating torque occurs and that sticking tendencies of the drill pipe are reduced. It is further known that this lubricating effect decreases with time, presumably because of chemical disintegration of the nut hulls in the circulating drilling fluid. It has been further shown in the prior art, specifically U.S. Pat. No. 2,943,679, Table V, that the lubricity effect of walnut shells is maximum in the 4 to 10 mesh size range and diminishes rapidly in sizes below 80 mesh.
Unfortunately, the use of ground walnut hulls necessitates the by-passing of the mud screens resulting in an undesirable build-up of clay solids in the drilling fluid. Furthermore, the incorporation of walnut hulls in the drilling fluid results in an increase in pump pressure, sometimes to such an extent that formations may be fractured thus inducing lost circulation. Both high clay solids and walnut hulls act to decrease penetration rate.
The 1977-78 Guide to Drilling, Workover and Completion Fluids, Gulf Publishing Company, Houston, Texas, lists some 62 proprietary drilling fluid lubricant additives offered by various drilling fluid additive suppliers. All of these compounds, composed of the above cited oils and soft solids lubricating materials, attest to the interest in, and need for, practical and effective means of reducing drag and rotating torque in the rotary drilling of wells.
The design and formulation of lubricant additives is made difficult by the fact that there are no standard methods of evaluating the effectiveness of such additives by laboratory tests.
Such testing was recently studied by a task group of the Committee on Standardization of Drilling Fluid Materials of the American Petroleum Institute. The variables involved, it was found, made such tests of little value in predicting the actual field performance of a given additive. Thus, despite of the obvious desirability of meaningful testing of lubricant additives, the task group was disbanded.
Regardless of the effectiveness of a drilling fluid lubricant in reducing friction in a laboratory friction test or in the field, the additive can be useful only if it meets criteria of practicality. It must not impair necessary drilling fluid properties of chemical or physical nature. It is recognized in the prior art that a lubricant may have limitations which seriously effect its usefulness. For example, a lubricant additive must have tolerance to the variation in pH and electrolytes normally encountered while drilling. Some additives curdle and ball up in the presence of calcium and are removed on the shale shaker screens. Other additives will cause oil wetting of barite in water base fluids or water wetting of barite in oil base fluids. In either case, the barite may objectionably settle out in low weight fluids or cause objectionably high viscosities in high weight fluids. Some additives cause foaming with the result that the mud becomes gas cut and unpumpable in the reciprocating mud pumps. Other additives resist wetting out and dispersion in the drilling fluid and float on the mud pits or are removed and discarded by the screens. Some additives fluoresce in ultraviolet light and thus interfere with certain well logging operations. Some of the proposed additives are effective only in uneconomical concentrations. Other additives may be potentially toxic, carcinogenic or environmentally undesirable.